Fast and accurate monitoring of the pH and toxic metals content of industrial waste streams and of waterways has become a task of paramount importance in current efforts to maintain the quality of the environment. Commercially available sensors for toxic metal ions are limited to ion-selective electrodes (ISEs) for detecting Ag(I), Cd(II), Cu(II), and Pb(II). Such devices are based on a potentiometric measurement, and their accuracy is affected by numerous interfering agents. Furthermore, ISEs are fragile, respond slowly, can be used only in aqueous solutions, are subject to fouling, and require frequent calibration. For these reasons ISEs are not suitable for widespread use in pollution-control systems, and most operators must therefore rely on costly and time-consuming off-site laboratory analyses to determine if a waste stream or groundwater contains excessive levels of toxic materials. Belyakov, in 17 Soviet Automatic Control 34 (1984), discloses several theoretical sensors comprising fibers, granules or round polymer contractile films operatively coupled to mechanical transducers such as springs and pistons. In discussing the theoretical kinetics of such sensors, the author assumed that the analyte of interest carries no electrical charge.
For on-site control of waste discharge, ideally a sensor would be connected to an automatic flow shut-off valve in the effluent stream by a feedback control loop such that if levels of hydrogen or hydroxide ion or certain metals exceeded preset limits, the sensor would signal the valve to interrupt discharge, thus preventing excessive release to the environment. In cases where contamination of the environment had already occurred or was suspected of occurring, remotely placed sensors could be used to continuously monitor the groundwater to provide a detailed map of the extent of contamination. For such applications a sensor must be sensitive to the analyte, must respond quickly to changes in the analyte concentration, must have a relatively long life, and must not require frequent calibration. These needs are met by the chemical sensor of the present invention.
Many polymers are known to be capable of exhibiting a physical response, such as expansion or contraction, to a change in the polymer's chemical environment, such as a change in the concentration of a given chemical species, usually as a result of chemical or physical interaction with the species. This capability is often referred to as "mechanochemical" responsiveness. Poly(methacrylic acid) (PMAA), crosslinked either with divinylbenzene or by esterification, is known to undergo volume expansions of up to 300% on conversion from the acid form to the polyanion form. Such expansion, which is reversible upon addition of mineral acid, has been explained on the basis of conformation changes in the polymer. The electrically neutral polyacid form consists of tightly coiled polymer chains. However, when the polymer is converted to the polyanion form, it has been theorized that electrostatic repulsion between negatively charged carboxylate groups results in full extension of the polymer chains and in the observed 300% expansion. See Kuhn et al., 7 Experientia 1 (1951).
This expansion/contraction response of methacrylic acid polymer is not limited to the reaction with hydrogen and hydroxide ions. PMAA and poly(acrylic acid) (PAA) have been shown to form complexes with transition metal and alkaline earth cations. See Gregor et al., 59 J. Phys. Chem. 34 (1955). PAA complexed more strongly than did PMAA, and both formed stronger complexes than did glutaric acid, their monomeric analog. The complexation constant varied with the metal and its valence; transition metals generally yielded stronger complexes than did alkaline earth metals, and divalent and trivalent ions complexed more strongly than did monovalent ions. See Osada, 18 J. Polym. Sci. 281 (1980). The experimental results suggest that the complexes involve two carboxylate ions per metal ion over a wide range of pH values. See Mandel et al., 2 J. Polym. Sci. 2883 (1964 part A). Crosslinked polymers formed weaker complexes than did linear polymers. Gregor et al., 59 J. Phys. Chem. 366 (1955). That the complexation of these metal ions by polycarboxylate anions involves a contractile response was demonstrated by the results of Osada, who used the same coiled-versus-extended-chain hypothesis to explain the increase in permeability observed in PMAA-grafted porous poly(vinyl alcohol) (PVA) membranes upon exposure to copper(II) salt solutions.
It has now been found that very sensitive, fast and long-lasting sensors for chemical species can be prepared by coupling an appropriate electrical transducer with thin polymer films that undergo a dimensional change in response to changes in the concentration of the species of concern.